Roof Truss Bracing
and Connections
What’s in this presentation
Types of temporary and permanent bracing
The problem of lateral buckling
Roof battens used to restrain top chords
The problem of roof plane distortion
Diagonal bracing for roof planes
Top chord bracing for different roof types
Bottom chord bracing
Web ties
Common truss connections
Typical girder bracket connections
Non-load bearing wall connections
Types of Bracing
Roof bracing stabilises the three dimensional roof structure. It also
transfers forces down to the structure below. There are two basic
types of bracing
Temporary - used during installation (refer module on “Roof truss
installation and site supervision” for details)
Permanent – used during the life of the building
Four types of permanent roof truss bracing described in AS4440
form the basis of discussion here:
Lateral restraint to prevent buckling of top chords
Diagonal bracing of top chords to stabilise roof planes
Lateral restraint to prevent buckling of bottom chords
Web ties
Each is discussed as follows
The Problem of Lateral Buckling
Downward loads on the roof cause compression in the top chords
Long slender members in compression tend to buckle sideways
Top chords are long slender compression members and therefore
tend to buckle sideways
compression
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Buckles
res
this way
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n
Roof Battens Used for Lateral
Restraint of Top Chords
Trusses need to be prevented from buckling as this causes them to
move outside their plane of strength (i.e. plane of vertical strength)
Fixing roof battens to the top chords at regular intervals acts to restrain
the sideways movement in the top chords – they should be fixed to
every truss including each ply of double and triple ply trusses.
Tip: Did you know special spacers provided by truss manufacturers may
be used to provide temporary lateral restraint before battens are placed
Interruptions to Roof Battens
Where saddle trusses overlap the main roof plane the continuity of
restraint may be jeopardised - trusses at wider spacings than the
intended restraints require intermediate top chord ties or battens
must continue through
For other roof planes interrupted by protrusion or overlapping roofs
(e.g. north light roofs) top chord ties are also needed - requirements
are specified in AS4440
Ref: Figure 1.6, AS4440 Copyright Standards Australia
The Problem of Roof Plane Distortion
Adding roof battens to restrain buckling only solves part of the
problem. Roof planes try to distort as a result of wind and other
forces
Although the battens connect all the top chords together the whole
thing can become distorted under wind or buckling actions – causing
the rectangles to become unwanted parallelograms
Using Diagonal Bracing to Prevent
Roof Plane Distortion
Diagonal bracing is required to address top chord distortion and is
used in other parts of the roof as well (e.g. bottom chords and webs)
It is often provided using “Steelbrace” – a product made from metal
strapping with a slight bend along the centre line
The Steelbrace only works in tension and is therefore applied in “X” or
“V” patterns across the roof planes (refer following slides).
This ensures that whichever way the loads act, the brace acts in tension.
A typical specification for Steelbrace is shown below
Copyright Standards Australia
Ref: Figure F1, AS4440
How the Diagonal
Bracing Works
Diagonal Steelbrace
crossing over the top chords
act as tension ties.
If the ties are only fixed at
the ends, the mid regions of
the top chords can still
move sideways
If the Steelbrace is fixed to
every element it crosses, the
forces arising from wind or
lateral buckling can be
transferred to the supporting
structure and the whole
assembly can be kept
square.
This is what we want for our
roof assemblies
Applying Diagonal Bracing as Top
Chord Bracing
Steelbrace is nailed to the top chords of trusses so that it makes a
series of “X” or “V” patterns across the roof planes. This ties and
assists the restraint provided by roof battens. The overall scheme is
shown in the figure below
Ref: Figure 4.1, AS4440
Copyright Standards Australia
Detail for Fixing Top Chord Bracing
to Walls
The Steelbrace must be attached to elements such as walls that
are also braced and capable of transferring loads down to the
structure below.
Ref: Figure 4.22, AS4440
Note: all nails
shown in drawing
to be Ø2.8mm x
30mm.
Copyright Standards Australia
Tip: Steelbrace fixing details for truss apexes, truss heels,
cantilevers, half trusses, splices and other situations also apply, and
can be found in AS4440
Example of Poor Installation
x
Bracing should
not be bent and
nailed into top of
top plate
Gun nails don’t have the
x reinforced head required to
prevent pull through
Detail for Fixing Top Chord Bracing
at Apexes
Ref: Figure 4.2.1, AS4440
Ref: Figure 4.1, AS4440
Copyright Standards Australia
Copyright Standards Australia
Example of Poor Installation
x
Bracing should
be wrapped
over face of
truss and nailed
Detail for Fixing Top Chord Bracing
at Splices
Ref: Figure 4.1, AS4440
Copyright Standards Australia
Specific Top Chord Bracing for
Gable Roofs
Australian Standard AS 4440 prescribes different versions of the
previously discussed “X” and “V” patterns depending on the load the
roof needs to resist and its dimensions - described in terms of roof
length
For gable roofs, the nature and frequency of bracing depends on three
things:
Roof Span – three span categories identified in AS4440 are:
Spans up to 8m
Spans 8 to 13m
Spans 13 to 16m
Roof Length - expressed as function of span. Four lengths categories
identified in AS4440 include:
Very short roofs (Roof length is 1 to 1.5 times the half span of the roof span)
Short roofs (Roof length is 1.5 to 3.5 times the half span of the roof span)
Long roofs (Roof length is 3.5 to 4 times the half span of the roof span)
Very long roofs (Roof length is more than 4 times the half span of the roof)
Roof wind load - high wind loads (especially cyclonic loads) may attract
double bracing instead of single bracing
Single bracing can be used for most spans approaching 13m but become
increasingly less applicable where larger spans coincide with high wind
loads and steep roof pitches. Double bracing is required in these instances.
Roof spans of 13 to 16m generally require double bracing except where
wind loads and roof pitches are relatively low
Ref: Figure 4.8, AS4440 Copyright Standards Australia
Examples of gable bracing for different roof
lengths in the 8-13m truss
span category Ref: Figure 4.12, AS4440
Ref: Figure 4.11, AS4440
Ref: Figure 4.10, AS4440
Copyright Standards Australia Copyright Standards Australia Copyright Standards Australia
Short roofs Long Roofs Very Long Roofs
Tip: Bracing for other spans and roof lengths are provided in AS4440
Specific Top Chord Bracing for Hip
and Dutch Gable Roofs
Hip and Dutch gable roofs require less bracing compared to
gables – they tend to self brace because of the sloping ends.
Even so, the same concepts of span and length determine the
nature and frequency of bracing.
Two spans categories are of particular interest: spans up to 13m,
and spans 13 to 16m
For spans up to 13m
Diagonal bracing is only required in the gable section between hip or Dutch
gable ends (as taken between the apex of opposing ends)
Specific bracing requirements depend on whether the roof length (L) is longer
or shorter than the half truss span (h). The shorter scenario is shown below
Ref: Figure 4.14b, AS4440 Copyright Standards Australia
Roof length L half span roof truss h
Tip: Bracing for other roofs in this range are provided in AS4440
For roof spans 13 - 16m, bracing is required on all hip or Dutch
gable ends. Specific requirements are determined by the horizontal
top chord length of the truncated girder (HTL), expressed as a ratio
of the truncated girder station (TGS) as follows:
HTL is less than the TGS;
HTL is 1 to 1.5 times greater TGS;
HTL is more than 1.5 times greater TGS (shown in the example below)
Ref: Figure 4.17, AS4440 Copyright Standards Australia
HTL > 1.5 x TGS
Tip: Bracing for other roofs in this range are provided in AS4440
Top Chord Bracing for Other Roofs
Roofs such as dual pitch, bell and mono-pitch, utilise the previous
themes of gable and hip bracing but add extra requirements dealing
with individual roof characteristics
AS4440 contains specific details and should be consulted as
required
Bottom Chord Bracing
Bottom chords need to be braced against lateral buckling and
distortion (just like top chords). It is the treatment of the ceiling
and its method of attachment to the bottom chords that dictate
bracing requirements. Three scenarios exist:
Ceilings fixed directly to the bottom chord -
Typically they require minimal bottom chord ties e.g. ties not
exceeding 4000mm apart
Battened ceiling
Ceiling battens are nailed or screwed to trusses and provide lateral
restraint - no additional restraint is required
Suspended or no ceiling (including the likes of clip on battens
that provide no lateral restraint)
Bottom ties must be used and must comply with an approved
specification (e.g. Ties capable of taking bracing loads; Steelbrace
used to prevent ceiling plane distortion)
Typical Bottom Chord Tie Bracing
Layout
Ref: Figure 4.28, AS4440 Copyright Standards Australia
Typical bottom chord bracing layout
Web Ties
The final consideration in bracing concerns web ties.
If the truss is heavily loaded and has deep slender web elements,
the designer may specify that the webs be laterally braced.
The timber web tie (shown) links the centre of all webs together (as
do battens for the top chords)
The Steelbrace stabilises the rectangular shape of the plane formed
by the webs.
Copyright Standards Australia
Ref: Figure 4.29, AS4440
Common Truss Connections
The main connections in trusses are factory fixed nailplates but many elements
in the roof assembly still require site fixing
AS4440 prescribes methods for common connections but in doing so, wind load
classifications dictate if skew nails, nail plates or metal strapping are used.
Click on the circles below to see connections suitable for low to mid range wind
classifications (N1,N2,N3, C1) applied to hip end connections. In high wind
areas (N4, C2 ,C3) the connections shown tend to be upgraded from nails to nail
plates, and nail plates to straps.
Ref: Figure 5.1, AS4440 Copyright Standards Australia
Ref: Excerpt from Figure 5.2, AS4440 Ref: Excerpt from Figure 5.2, AS4440
Copyright Standards Australia
or
Copyright Standards Australia
Fixing detail A1 Fixing detail B1
(for non-cyclonic and cyclonic wind classifications N1, N2, N3, C1)
Ref: Excerpt from Figure 5.2, AS4440 Ref: Excerpt from Figure 5.2, AS4440
or
Copyright Standards Australia Copyright Standards Australia
Fixing detail A1 Fixing detail E1
(for non-cyclonic and cyclonic wind classifications N1, N2, N3, C1)
Ref: Excerpt from Figure 5.2, AS4440
Copyright Standards Australia
Fixing detail B1
(for non-cyclonic and cyclonic wind
classifications N1, N2, N3, C1)
Ref: Excerpt from Figure 5.2, AS4440
Copyright Standards Australia
Fixing detail C1
(for non-cyclonic and cyclonic wind
classifications N1, N2, N3, C1)
Ref: Excerpt from Figure 5.2, AS4440 Ref: Excerpt from Figure 5.2, AS4440
or
Copyright Standards Australia Copyright Standards Australia
Fixing detail D1 Fixing detail E1
(for non-cyclonic and cyclonic wind classifications N1, N2, N3, C1)
Example of Poor Installation
x
Nail missing from
nail plate – the
manufacturer’s
specified nailing
requirements must
be followed
Typical Girder Bracket Connections
Girder brackets (or truss boots) transfer loads from standard trusses
to girder trusses. The load involved makes this connection
important.
Different bracket options exist depending on load requirements.
Ref: Figure G.1, AS4440
Framing bracket
(joist hanger) can support
small span standard
trusses
Copyright Standards Australia
Ref: Figure G.3, AS4440
Anti-twist truss boots
support standard trusses,
truncated standard
trusses and small girder
trusses
Copyright Standards Australia
Ref: Figure G.4, AS4440
High load truss boots are
usually made from
welded brackets. They
support large standard,
and truncated standard
trusses, as well as girder
and truncated girder
trusses
Copyright Standards Australia
Example of Poor Installation
x
Screws should
be in each of the
small holes.
Non-load bearing wall Connections
External walls are usually used to provide support for trusses but
internal walls are usually non-load bearing
Internal walls cause problems if used for support because they
change the way the truss works. To prevent this:
External load bearing walls are made slightly higher than internal walls,
leaving a gap between the bottom chord and the internal wall
Special brackets fix the bottom chord to the internal wall – the brackets
allow the bottom chord to move up and down in the gap (but not
sideways)
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